Modern wind turbines which are capable of delivering a plurality of megawatts of electric power and which are also known as multi-megawatt turbines comprise rotor blades which are rotatably mounted on a rotor so that by changing the blade angle relative to the rotor, the angle of incidence of the wind can be varied for each rotor blade.
According to a first method for operating a wind turbine, a single blade angle error is delivered to all rotor blade control means so that a common change in the angle of all the rotor blades takes place. On the other hand, according to a second method for operating a wind turbine, an individual blade angle control means is used, also known as IPC, so that the blade angles for each rotor blade are adjusted individually. In an ideal case, the use of an IPC can reduce the energy costs, since it is possible to either reduce the initial investment costs as a result of mass reduction of structural elements of the wind turbine or the annual energy production can be increased due to longer rotor blades.
However, the IPC is not a mature technology, but is the subject of ongoing developments. The objective of these developments is to reduce loads, arising temporarily on the rotor blades, due to shear stresses and wind turbulence. Known control strategies which use the IPC to reduce such loads are either based on the evaluation of load, expansion or acceleration signals obtained from one or a plurality of rotor blades or on the evaluation of deformations of the main shaft, supporting the rotor, of the wind turbine.
U.S. Pat. No. 6,361,275 B1 discloses an individual blade angle control means which is based on blade load sensors or on wind speed sensors on the blade surface, to reduce loads caused by local, temporary peaks of the wind speed in parts of the rotor region.
WO 01/33075 A1 describes an individual blade angle control means which is based on mechanical loads on the rotor blades in order to operate the wind turbine closer to its design limits, without exceeding these limits.
According to WO 2004/074681 A1, to improve the stability of the wind turbine, flow characteristics are measured locally in front of each rotor blade (for example by a long arm which extends away forwards from the blade and supports an anemometer), so that fatigue loads and the risk of rotor blade-tower interactions are reduced.
WO 2008/041066 A1 discloses an individual blade angle control means to reduce moments caused by wind shear and maladjustments of the yaw angle, the blade angle being controlled subject to measured moments acting on the rotor blades to reduce the difference of the measured moments from a set value (which is stored in a memory).
WO 2008/087180 A2 describes an individual blade angle control means for reducing asymmetric loads on the rotor, the blade angle being controlled subject to measured deformations of the main shaft (measurement using strain gauges).
U.S. Pat. No. 7,118,339 B2 discloses an individual blade angle control means and a yaw angle control means which are based on rotor blade load measurements and respectively on main shaft displacement measurements, so that the wind turbine is brought into a favourable alignment by yawing, the individual blade angle control means being used to reduce additional asymmetric rotor loads (i.e. fatigue loads). Furthermore, the operation of the wind turbine with a predetermined yaw angle error is disclosed to counteract asymmetric rotor loads.
WO 2008/119351 A2 describes an individual blade angle control means to generate a rotor moment which counteracts the moment of gravity so that the loads on the main shaft bearings are reduced.
The above-mentioned documents attempt to indirectly lower the energy costs by reducing the fatigue loads so that it is possible to reduce the structural loading capacity of the wind turbine (for example by reducing the weight). In order to be able to allow a reduction of the structural loading capacity, other loads such as extreme weather conditions to which the wind turbine is exposed during its service life also have to decrease. Otherwise, the advantages which can be achieved with the IPC are partly or completely lost. For these and other reasons, the IPC is currently not used on a commercial level.
Thus, there is a need for an IPC system which can reduce the energy costs and which can be used with a current wind turbine design with minimum modifications to the construction of the wind turbine.
Another field in connection with the operation of wind turbines but which is not the subject of the above-mentioned documents relates to the effect of yaw angle errors. The yaw angle error is defined as the angle between the wind direction and the rotor axis. However, the vertical component of the wind speed is not considered when determining the yaw angle error because the inclination of the rotor axis cannot be altered during operation of the wind turbine (an upwards incline of the rotor axis of, for example 5° is typical of commercial wind turbines). Therefore, to determine the yaw angle error, only the component, located in the horizontal plane, of the wind speed is considered. Yawing the rotor of the wind turbine can reduce the yaw angle error to zero (with constant wind conditions).
WO 2008/143009 A1 discloses an individual blade angle control means for generating forces which engage on the rotor blades and generate a yaw moment for yawing the wind turbine. It is thus possible to partly or completely dispense with yaw drives, thus making it possible to reduce electrical power losses. However, the availability of forces engaging on the rotor blades for generating the yaw moment is subject to the sporadic and stochastic nature of the wind, including the effect of turbulence, so that precise and timely yaw procedures are impossible. In order to achieve a controllable yaw movement based on these forces, the yaw movement is damped by a braking system during the entire yaw procedure. However, this damping significantly reduces the velocity of the yaw movement so that the friction forces of the braking system have to be overcome by increased, cyclic blade loads, which is detrimental to the service life of the wind turbine and is thus undesirable. Furthermore, the additional electrical power loss which is associated with the increased activity of the blade angle adjusting drive cancels out the advantages which are associated with the lower power loss of the yaw drive. Apart from this, when there is a lull in the wind, the wind turbine must be able to yaw to unravel cables, so that the desire to eliminate the electrical yaw system cannot be realised in practice.
Due to undesirable and damaging gyroscopic rotor blade loads which arise during yawing, the yaw rate of present commercial multi-megawatt wind turbines is below a value of 0.7°/s (in most cases even below 0.5°/s). These gyroscopic loads increase linearly with the yaw rate so that a restriction of the yaw rate also restricts the gyroscopic loads. The yaw rate is also called yaw angular velocity.
Detailed measurements of the yaw angle error on ready-for-use, commercial multi-megawatt wind turbines, as described, for example, in the report Risø-R-1654 (EN) by T. F. Pedersen, N. N Sørensen, L. Vita and P. Enevoldsen (2008) entitled “Optimization of Wind Turbine Operation By Use of Spinner Anemometer” show that the yaw angle error attains instantaneous values of more than 30°, and that deviations of up to 4°/s occur over a significant period of time. This angular velocity is significantly above the limit of 0.4°/s to 0.6°/s of current commercial multi-megawatt wind turbines. Consequently, current wind turbines cannot track the changes in wind direction and are continuously operated under significant yaw angle errors. Typical wind turbine control means allow a yaw angle error of approximately 25° to 30° when the yaw angle error is averaged over a period of 5 to 15 seconds, a yaw angle error of 10° to 15° when the yaw angle error is averaged over one minute and a yaw angle error of 3° to 6° when the yaw angle error is averaged over 10 minutes or more.
Thus, there is a need for a facility to be able to continuously operate modern multi-megawatt wind turbines with reduced or small yaw angle errors. Reducing the yaw angle error permits increased energy consumption if the wind turbine is operated below the nominal speed. Furthermore, a reduction in the yaw angle error reduces flexural loads on the rotor blades which are caused by asymmetric wind conditions over the area covered by the rotor if the wind turbine is operated above the nominal speed.
The prior art solutions which use individual blade angle control means based on blade loads or blade accelerations cannot be used successfully to reduce gyroscopic loads. On the one hand, wind turbulence causes changes in the blade loads and blade accelerations, thereby concealing the onset of gyroscopic loads. On the other hand, the gyroscopic blade loads lag (temporally) behind the blade angle adjustment due to the inertia of the rotor blades. In both cases, this leads to a delayed and ineffective blade angle adjustment with respect to the reduction of gyroscopic loads.
US 2009/0068013 A1 discloses a method for reducing loads which act on a yaw system of a wind turbine due to yaw moments, the yaw moments being introduced into the yaw system by a rotor which comprises a rotor blade with a blade angle adjusting system. The yaw moment introduced into the yaw system by the rotor is determined and, based on the ascertained yaw moment, a blade angle of the rotor blade is adjusted such that the determined yaw moment is reduced.
According to this method, it is not the gyroscopic loads, caused by a yaw procedure, of the individual rotor blades, but the moments which act around the yaw axis and engage on the yaw system which are reduced. In particular, aerodynamic loads of the yaw system are to be reduced as a result of rotating the rotor blades about their blade axes subject to a set value for the yaw moment.